In principle there are two main types of wind turbines in view of the drive configuration of a wind turbine. The first type of a wind turbine is the more classical type of a wind turbine comprising a gearbox arranged between a main shaft and a generator of the wind turbine. The second type of a wind turbine is a gearless type, whereat the gearbox and the conventional generator are substituted by a multipolar generator, a so called direct drive or directly driven generator. Such a direct drive generator can be made as a synchronous generator with winded rotor or with permanent magnets attached to the rotor, or it can be designed as an alternative type of a generator.
Common to direct driven generators is that their physical dimensions are relatively large. At a typical air gap diameter of approximately 5 m for a multi-megawatt direct drive generator the outer diameter is on the order of approximately 6 m or even more. The large outer diameter makes the transport of the direct drive generator difficultly and the heavy dead load of the direct drive generator involves further difficulties e.g. concerning the replacement for repair by occurred breakdowns.
A further difficulty arises in the normal configuration of a wind turbine with a direct drive generator, where the direct drive generator is arranged between the wind turbine rotor and the tower in order to yield a compact machine construction. In this case it will be necessary to dismantle the whole wind turbine rotor by a required dismantling of the direct drive generator.
To overcome these problems at least partially there were some suggestions to divide parts of the generator.
In WO 98/20595 A1 a stator for a rotating electric machine is disclosed comprising a stator core and a winding. The stator core is provided with stator teeth extending radially inwards towards a rotor. Each stator tooth is configured as a number of tooth sections joined axially into a stator tooth plank. That stator tooth planks are fitted together side by side thus forming a section of the stator core. This construction makes the transport of parts of the rotating electric machine to the site of erection partially easier, because the stator can be assembled on site. However, this construction requires a stator housing as such having relatively large outer dimensions.
From U.S. Pat. No. 4,594,552 an armature winding of a split stator is known. The split stator has a slotted core divided by at least two circumferentially-spaced split lines to facilitate the assembly and the disassembly of the split stator. The armature winding comprises armature coils in the slots of the stator core connected to provide poles and arranged to provide a plurality of armature coils divided at the split lines. Connecting and disconnecting means are provided to connect and disconnect the armature coils when the split stator is assembled and disassembled, respectively. This construction, however, also requires a stator housing as such having relatively large outer dimensions.
U.S. Pat. No. 5,844,341 describes an electric generator to be driven by a low speed device such as wind turbine. The generator consists of one or more rotor rings of many permanent magnets of alternating polarity and coaxial stator rings of many laminated yokes, each yoke defining slots to locate coils. The yokes and coils form modules which are supported by beams relatively to the rotor rings. The drawback of this configuration is that the electromechanical properties in this form of modular construction with single polar pairs separated by air gaps may be disadvantageous, and that a possible dismantling of a single stator module can involve that the whole generator has to be opened in situ implying risk of humidity, dirt etc., and that it may be cumbersome if the stator module has to be taken out in a disadvantageous direction.
U.S. Pat. No. 6,781,276 B1 describes a generator for a wind turbine comprising a stator and a rotor. The stator has a number of stator modules that are individual and which may be installed, repaired and dismantled individually and independently of each other. This generator has no part larger than the air gap diameter. But even if no part is larger than the air gap diameter, the largest element to be transported still has a substantial size, given that the rotor is a single piece. In its completed form this rotor is fitted with strong permanent magnets and needs to be covered by a nonmagnetic layer, e.g. wood or polystyrene of a certain thickness during transportation, and while the dimensions of the rotor are smaller than the dimensions of the finished generator, it is still at maybe 5 m diameter and 1.5 m length a very substantial piece of equipment to transport.